The spare could be secured behind the rear seat, which can be slid pretty close to the hatch, or on top between the crossbars of the roof rack. A Chrysler dealer may have someone who can answer your question about whether it can be carried underneath. I am curious about it, too, and wish you luck.
My wife and I like the shorter Chrysler vans, which they dropped for the 2008s, so we found a decent used 2007 SWB T&C to replace our rusty 1999 Plymouth Voyager.